Thermal transfer recording is generally carried out by heating a thermal transfer recording material, called an ink ribbon, which comprises a support having thereon a color forming layer containing a sublimable or vaporizable dye to sublimate or vaporize the dye and then transferring the dye to an image-receiving sheet to form a dye image.
More specifically, as shown in FIG. 3, thermal transfer recording material 1 composed of support 4 and color forming layer 5 and image-receiving sheet 2 composed of thermal transfer image-receiving layer 6 and support 7 are brought into contact between drum 12 and electrically controlled heating source 3, and color forming layer 5 of transfer recording material 1 is heated by means of heat source 3, such as a thermal head, to sublimate or vaporize the dye contained in color forming layer 5. The sublimated or vaporized dye is thus transferred to image-receiving layer 6 to achieve thermal transfer recording.
The material constituting image-receiving layer 6 depends on the kind of the color former (dye) to be transferred thereto. For example, if one uses a heat-fusible color former, support 7 by itself can serve as an image-receiving layer. If one uses a sublimable disperse dye as a color former, a coated layer comprising a high polymer, such as a polyester, may be used as an image-receiving layer.
Support 7 of image-receiving sheet 2 typically will be a pulp paper, an opaque synthetic paper comprising a stretched film of a propylene-based resin containing an inorganic fine powder (see JP-B-46-40794 (corresponding to U.S. Pat. No. 4,318,950), the term "JP-B" as used herein means an "examined published Japanese patent application"), or a coated synthetic paper prepared by coating a transparent polyethylene terephthalate or polyolefin film with a layer of a binder containing an inorganic Compound, such as silica or calcium carbonate, to impart whiteness and dye-receptivity thereto.
Considering the after-use properties of an image-receiving sheet with a transferred dye image with respect to, for example, suitability for copying, writability with a pencil, and record preservability, a synthetic paper comprising a microvoid-containing stretched film of a polyolefin resin containing an inorganic fine powder is preferred as a support from the standpoint of strength, dimensional stability, and contact with a printing head, as disclosed in JP-A-60-245593, JP-A-61-112693 and JP-A-63- 193836 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In this type of synthetic paper, microvoids are formed by stretching an inorganic fine powder-containing polyolefin resin film at a temperature lower than the melting point of the polyolefin resin so as to provide opacity, softness to the touch, intimate contact with a printing head, and smoothness in paper feed or discharge.
In recent years, a demand has arisen for thermal transfer image-receiving sheets with high surface gloss. In order to increase gloss, it is desirable to use a synthetic paper having a surface layer containing substantially no inorganic filler. However, even if an image-receiving sheet itself has gloss, if the support thereof has a rough feeling due to surface unevenness, the glossy feeling of a transferred image will be impaired.
Further, with the latest rapid advances in the speed of printing with a thermal transfer recording apparatus, the industry has demanded images of high density with satisfactory gradation even with a narrowed pulse width, particularly with a thermal transfer image-receiving sheet capable of multiple transfer as disclosed in JP-A-63-222891.
It is a common knowledge in the art that printing density can be increased by increasing the surface smoothness of an image-receiving sheet. If the compounding ratio of inorganic fine powder is reduced in an attempt to increase the surface smoothness of a synthetic paper support, the voids formed by stretching will be reduced in number, resulting in a reduction in the cushioning effect of the support. It follows that image density is then reduced as is established in Comparative Example 1 of JP-A-63-222891 supra.